El Moro canyon is a good place for a hike. I run those trails on the weekend from the top of pelican hill down to the beach. Beautiful views, very secluded, and no noise from cars once you get away from the 73.
Also, if you're into climbing, or want to get into it, check out Rockreation in Costa Mesa. Good if your new or old to the sport. They change up the problems frequently to keep it interesting.
Like Havasu said, the Orange County Marketplace at the fairgrounds is a good way to kill a weekend afternoon (and there's always other stuff going on there, too). There's also a great farmer's market culture developing, serviced by live music and EXCELLENT food trucks, examples in several cities. No one's mentioned the obvious yet: THE BEACH. Hiking in South County's been mentioned. I'll add in the Santa Ana river trail. Most of it's just concrete, but the last few miles before the ocean are awesome (along with the nature preserve and Fairview Park) and, at the other end, the last few miles before crossing the SB county line are cool, too.